Say you have a very attractive sister and your best friend wanted to date her, is he wrong for asking? Is he not supposed to? Thoughts......
Let me turn this around for you. IF your best friend had a sister you COULD date who was a knock out do you think the fact that he's your BF should be an obstacle? This is not to say that IF you were to ask her out, you shouldn't refuse to tell him any "details". That however is the one real complication I can see. In the reverse, unless he does (or would) treat her really wrong what's the problem? SOMEONE is going to date her, why shouldn't it be your buddy if she's interested?
Back in the day, where I group up, that was something you simply did not do. Would I today? I honestly don't know.
Thanks Wardog that's a really good response, however I could of used it earlier, say a few years my result I was beaten up by three guys for at least asking if it was okay if I asked my best buddies sister out.
I guess I am old school. NO. That is just something that you don't do. Kind of like you don't date a best friend's ex (or your brother's ex).